Direct methanol feed fuel cell and system

ABSTRACT

Improvements to non-acid methanol fuel cells include new formulations for materials. The platinum and ruthenium are more exactly mixed together. Different materials are substituted for these materials. The backing material for the fuel cell electrode is specially treated to improve its characteristics. A special sputtered electrode is formed which is extremely porous.

This is a divisional of U.S. application Ser. No. 09/006,846, filed Jan.14, 1998 (now U.S. Pat. No. 6,146,781); which is a continuation of U.S.application Ser. No. 08/569,452, filed Dec. 8, 1995 (now U.S. Pat. No.5,773,162); which is a continuation of U.S. application Ser. No.08/135,007, filed Oct. 12, 1993 (now U.S. Pat. No. 5,599,638); which isa continuation-in-part of U.S. application Ser. No. 08/478,001, filedJun. 7, 1995 (now U.S. Pat. No. 5,645,573).

FIELD OF THE INVENTION

The present invention relates to direct feed methanol fuel cellimprovements for a system that operates without an acid electrolyte or areformer.

BACKGROUND AND SUMMARY

Transportation vehicles which operate on gasoline-powered internalcombustion engines have been the source of many environmental problems.The output products of internal combustion engines cause, for example,smog and other exhaust gas-related problems. Various pollution controlmeasures minimize the amount of certain undesired exhaust gascomponents. The process of burning, however, inherently produces someexhaust gases.

Even if the exhaust gases could be made totally benign, however, thegasoline based internal combustion engine still relies on non-renewablefossil fuels.

Many groups have searched for an adequate solution to the energyproblems.

One possible solution has been fuel cells. Fuel cells chemically reactusing energy from a renewable fuel material. Methanol, for example, is acompletely renewable resource. Moreover, fuel cells use anoxidation/reduction reaction instead of a burning reaction. The endproducts from the fuel cell reaction are typically mostly carbon dioxideand water.

Some previous methanol fuel cells used a “reformer” to convert themethanol to H₂ gas for a fuel cell. Methanol fuel cells used a strongacid electrolyte. The present inventors first proposed techniques whichwould allow a fuel cell to operate directly from methanol and without anacid electrolyte—a direct feed fuel cell. The subject matter of thisimprovement is described in our U.S. Pat. No. 5,599,638, the disclosureof which is herewith incorporated by reference to the extent necessaryfor proper understanding. Since this is the work of the presentinventors, of course, there is no admission made here that this patentconstitutes prior art against the present invention.

The subject matter of the present invention describes furtherrefinements of such a direct fed fuel cell. Various improvements to thefuel cell structure itself are described herein, based on the inventors'further work on this concept. These improvements include improvedformulations for the electrode which improve its operation.

The electrode operation includes an improved catalyst, which improvesthe efficiency of methanol production. Fuel cells use an expensiveplatinum catalyst. The electrode formulations given herein definetechniques which reduce or obviate the need for the platinum catalyst.

Techniques for forming the cathode electrode are also described herein.These techniques optimize the operation of the cathode for use withnon-pressurized air. This even further improves the efficiency of thefuel cell by allowing ambient temperature and atmospheric pressure airas the reduction mechanism.

Formation techniques for the electrodes are also described, includingtechniques to condition the membrane. A formation of a particularlypreferred membrane electrode assembly is also defined.

The present invention also defines flow field designs which facilitatesupplying the liquid fuel to the catalyst.

The fuel cell system eventually needs to be used in a final product.This final product could be an internal combustion engine or could bemuch simpler electronic devices, such as a radio. Anyelectrically-driven product could operate based on electrical powerproduced from these fuel cells. The inventors of the present inventionhave discovered certain techniques to improve the operation andameliorate these problems which might otherwise exist.

The techniques of the present invention also enable a “system operation”by describing techniques to operate the fuel cell as part of an overallsystem.

These system techniques includes sensors for measuring methanolconcentration and other important parameters. The inventors realizedthat various sensors for various parameters would be necessary. Theinventors could not find a commercial sensor. The present inventiondescribes a way of modifying the techniques which they use in their fuelcell to form a sensor. This sensor operates with high reliability usingthe techniques of this fuel cell.

Another technique defines formation of monopolar cells.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be described in detailwith reference to the accompanying drawings, wherein:

FIGS. 1 and 2 show a basic fuel cell according to the present invention;

FIG. 3 shows the drying dish used for drying teflon encoded carbon papersheets;

FIG. 4 shows the basic platinum sputtering device of the presentinvention;

FIG. 5 shows a basic flow field apparatus according to the firstembodiment of the present invention;

FIG. 6 shows a cross-sectional view of the preferred flow field alongthe line 66 in FIG. 5;

FIG. 7 shows a first embodiment of the structure of the biplate of thepresent invention;

FIG. 8 shows a second embodiment of the biplate structure;

FIG. 9 shows a system operation of the direct methanol field fuel cell;

FIG. 10 shows how the fuel cell concepts described above would bemodified for use in a methanol sensor;

FIG. 11 shows the methanol concentration versus current relationship ofthe present invention;

FIG. 12 shows a graded molecular sieve fuel cell for methanol accordingto the present invention;

FIG. 13 shows a first, expanded, figure of a monopolar approach to afuel cell of the present invention;

FIG. 14 shows the packaging of this monopolar approach;

FIG. 15 shows a second embodiment of the monopolar approach in expandedview;

FIG. 16 shows how this monopolar approach would be assembled into anoperating system; and

FIG. 17 shows the different expanded layouts of the monopolar approachassembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The liquid feed system described in our above noted patent uses aplatinum-ruthenium catalyst on the anode and a platinum catalyst on thecathode. A perfluorsulfonic acid membrane, preferably DuPont's NAFION™117 tetrafluoroethylene-perfluorovinyl ether sulfonic acid copolymer, isused as the polymer electrolyte membrane.

Importantly, this system operated without the necessity for any acidelectrolyte, or reformer. Various characteristics of various materialswere changed to allow this improvement.

The anode electrode was made more hydrophilic using an ionomericadditive to improve access of the methanol water solutions to the anodesurface.

An alternative way of making the anode more hydrophilic was to use anelectrolyte which included a super acid (“a C8 acid”).

Alternative methanol derivative fuels, such as trimethoxymethane “TMM”,reduce fuel crossover due to their increased molecule size, and othercharacteristics.

FIG. 1 illustrates a liquid feed organic fuel cell 10 having a housing12, an anode 14, a cathode 16 and a solid polymer proton-conductingcation-exchange electrolyte membrane 18. As will be described in moredetail below, anode 14, cathode 16 and solid polymer electrolytemembrane 18 are preferably a single multi-layer composite structure,referred to herein as a membrane-electrode assembly. A pump 20 isprovided for pumping an organic fuel and water solution into an anodechamber 22 of housing 12. The organic fuel and water mixture iswithdrawn through an outlet port 23 and is re-circulated through are-circulation system described below with reference to FIG. 2 whichincludes a methanol tank 19. Carbon dioxide formed in the anodecompartment is vented through a port 24 within tank 19. An oxygen or aircompressor 26 is provided to feed oxygen or air into a cathode chamber28 within housing 12. FIG. 2, illustrates a fuel cell systemincorporating a stack 25 of individual fuel cells including there-circulation system, which includes a heat exchanger 37 receiving theoutput from the anode outlet port 23 of the stack 25, fuel/watercirculation tank 35 and a pump 20 to inject a fuel and water solutioninto the anode chamber 22 of the stack 25. Methanol from a methanolstorage tank 33 enters a fuel and water injection system 29, whichprovides an input stream to the circulation tank 35. An oxidant supplysystem 26 supplies air or oxygen to the cathode chamber 28 of the stack25. Carbon dioxide and water emitted from the cathode chamber 28 atoutlet 30 are provided to a water recovery unit 27, which in turnsupplies water to the fuel/water injection system 29. The followingdetailed description of the fuel cell of FIG. 1 primarily focuses on thestructure and function of anode 14, cathode 16 and membrane 18.

Prior to use, anode chamber 22 is filled with the organic fuel and watermixture and cathode chamber 28 is filled with air or oxygen. Duringoperation, the organic fuel is circulated past anode 14 while oxygen orair is pumped into chamber 28 and circulated past cathode 16. When anelectrical load (not shown) is connected between anode 14 and cathode16, electro-oxidation of the organic fuel occurs at anode 14 andelectro-reduction of oxygen occurs at cathode 16. The occurrence ofdifferent reactions at the anode and cathode gives rise to a voltagedifference between the two electrodes. Electrons generated byelectro-oxidation at anode 14 are conducted through the external load(not shown) and are ultimately captured at cathode 16. Hydrogen ions orprotons generated at anode 14 are transported directly across membraneelectrolyte 18 to cathode 16. Thus, a flow of current is sustained by aflow of ions through the cell and electrons through the external load.

As noted above, anode 14, cathode 16 and membrane 18 form a singlecomposite layered structure. In a preferred implementation, membrane 18is formed from NAFION™ tetrafluoroethylene-perfluorovinyl ether sulfonicacid copolymer, a perfluorinated proton-exchange membrane material.NAFION™ tetrafluoroethylene-perfluorovinyl ether sulfonic acid copolymeris a co-polymer of tetrafluoroethylene and perfluorovinylether sulfonicacid. Other membrane materials can also be used. For example, membranesof modified perflourinated sulfonic acid polymer, polyhydrocarbonsulfonic acid and composites of two or more kinds of proton exchangemembranes can be used.

Anode 14 is formed from platinum-ruthenium alloy particles either asfine metal powders, i.e. “unsupported”, or dispersed on high surfacearea carbon, i.e. “supported”. The high surface area carbon may bematerial such as VULCAN XC-72A carbon, provided by Cabot Inc., USA. Acarbon fiber sheet backing (not shown) is used to make electricalcontact with the particles of the electrocatalyst. Commerciallyavailable TORAY™ carbon fiber paper paper is used as the electrodebacking sheet. A supported alloy electrocatalyst on a TORAY™ carbonfiber paper paper backing is available from E-Tek, Inc., of Framingham,Mass. Alternately, both unsupported and supported electrocatalysts maybe prepared by chemical methods, combined with TEFLON™polytetrafluoroethylene binder and spread on TORAY™ carbon fiber paperpaper backing to produce the anode. An efficient and time-savingpreferred method of fabrication of electro-catalytic electrodes isdescribed in detail hereinbelow.

Platinum-based alloys in which a second metal is either tin, iridium,osmium, or rhenium can be used instead of platinum-ruthenium. Ingeneral, the choice of the alloy depends on the fuel to be used in thefuel cell. Platinum-ruthenium is preferable for electro-oxidation ofmethanol. For platinum-ruthenium, the loading of the alloy particles inthe electrocatalyst layer is preferably in the range of 0.5-4.0 mg/cm².More efficient electro-oxidation is realized at higher loading levels,rather than lower loading levels.

Cathode 16 is a gas diffusion electrode in which platinum particles arebonded to one side of membrane 18. Cathode 16 is preferably formed fromunsupported or supported platinum bonded to a side of membrane 18opposite to anode 14. Unsupported platinum black (fuel cell grade)available from Johnson Matthey Inc., USA or supported platinum materialsavailable from E-Tek Inc., USA are suitable for the cathode. As with theanode, the cathode metal particles are preferably mounted on a carbonbacking material. The loading of the electrocatalyst particles onto thecarbon backing is preferably in the range of 0.5-4.0 mg/cm². Theelectrocatalyst alloy and the carbon fiber backing contain 10-50 weightpercent TEFLON™ polytetrafluoroethylene to provide hydrophobicity neededto create a three-phase boundary and to achieve efficient removal ofwater produced by electro-reduction of oxygen.

During operation, a fuel and water mixture (containing no acidic oralkaline electrolyte) in the concentration range of 0.5-3.0 mole/literis circulated past anode 14 within anode chamber 22. Preferably, flowrates in the range of 10-500 ml/min. are used. As the fuel and watermixture circulates past anode 14, the following electro-chemicalreaction, for an exemplary methanol cell, occurs releasing electrons:

Anode: CH₃OH+H₂O→CO₂+6H⁺+6e⁻  (1)

Carbon dioxide produced by the above reaction is withdrawn along withthe fuel and water solution through outlet 23 and separated from thesolution in a gas-liquid separator (See FIG. 2). The fuel and watersolution is then re-circulated into the cell by pump 20.

Simultaneous with the electrochemical reaction described in equation 1above, another electrochemical reaction involving the electro-reductionof oxygen, which captures electrons, occurs at cathode 16 and is givenby:

Cathode: O₂+4H⁺+4e⁻→H₂O  (2)

The individual electrode reactions described by equations 1 and 2 resultin an overall reaction for the exemplary methanol fuel cell given by:

Cell: CH₃OH+1.50₂→CO₂+2H₂O  (3)

At sufficiently high concentrations of fuel, current densities greaterthan 500 mA/cm can be sustained. However, at these concentrations, acrossover rate of fuel across membrane 18 to cathode 16 increases to theextent that the efficiency and electrical performance of the fuel cellare reduced significantly. Concentrations below 0.5 mole/liter restrictcell operation to current densities less than 100 mA/cm². Lower flowrates have been found to be applicable at lower current densities. Highflow rates are required while operating at high current densities toincrease the rate of mass transport of organic fuel to the anode as wellas to remove the carbon dioxide produced by electrochemical reaction.Low flow rates also reduce the crossover of the fuel from the anode tothe cathode through the membrane.

Preferably, oxygen or air is circulated past cathode 16 at pressures inthe range of 10 to 30 psig. Pressures greater than ambient improve themass transport of oxygen to the sites of electrochemical reactions,especially at high current densities. Water produced by electrochemicalreaction at the cathode is transported out of cathode chamber 28 by flowof oxygen through port 30.

In addition to undergoing electro-oxidation at the anode, the liquidfuel which is dissolved in water permeates through solid polymerelectrolyte membrane 18 and combines with oxygen on the surface of thecathode electrocatalyst. This process is described by equation 3 for theexample of methanol. This phenomenon is termed “fuel crossover”. Fuelcrossover lowers the operating potential of the oxygen electrode andresults in consumption of fuel without producing useful electricalenergy. In general, fuel crossover is a parasitic reaction which lowersefficiency, reduces performance and generates heat in the fuel cell. Itis therefore desirable to minimize the rate of fuel crossover. The rateof crossover is proportional to the permeability of the fuel through thesolid electrolyte membrane and increases with increasing concentrationand temperature. By choosing a sold electrolyte membrane with low watercontent, the permeability of the membrane to the liquid fuel can bereduced. Reduced permeability for the fuel results in a lower crossoverrate. Also, fuels having a large molecular size have a smaller diffusioncoefficient than fuels which have small molecular size. Hence,permeability can be reduced by choosing a fuel having a large molecularsize. While water soluble fuels are desirable, fuels with moderatesolubility exhibit lowered permeability. Fuels with high boiling pointsdo not vaporize and their transport through the membrane is in theliquid phase. Since the permeability for vapors is higher than liquids,fuels with high boiling points generally have a low crossover rate. Theconcentration of the liquid fuel can also be lowered to reduce thecrossover rate. With an optimum distribution of hydrophobic andhydrophilic sites, the anode structure is adequately wetted by theliquid fuel to sustain electrochemical reaction and excessive amounts offuel are prevented from having access to the membrane electrolyte. Thus,an appropriate choice of anode structures can result in the highperformance and desired low crossover rates.

Because of the solid electrolyte membrane is permeable to water attemperatures greater than 60° C., considerable quantities of water aretransported across the membrane by permeation and evaporation. The watertransported through the membrane is condensed in a water recovery systemand fed into a water tank (See FIG. 2) so that the water can bere-introduced into anode chamber 22.

Protons generated at anode 14 and water produced at cathode 16 aretransported between the two electrodes by proton-conducting solidelectrolyte membrane 18. The maintenance of high proton conductivity ofmembrane 18 is important to the effective operation of an organic/airfuel cell. The water content of the membrane is maintained by providingcontact directly with the liquid fuel and water mixture. The thicknessof the proton-conducting solid polymer electrolyte membranes shouldpreferably be in the range from 0.05-0.5 mm. Membranes thinner than 0.05mm may result in membrane electrode assemblies which are poor inmechanical strength, while membranes thicker than 0.5 mm may sufferextreme and damaging dimensional changes induced by swelling of thepolymer by the liquid fuel and water solutions and also exhibitexcessive resistance. The ionic conductivity of the membranes should begreater than 1 ohm⁻¹ cm⁻¹ for the fuel cell to have a tolerable internalresistance.

As noted above, the membrane should have a low permeability to theliquid fuel. Although a NAFION™ tetrafluoroethylene-prefluorovinyl ethersulfonic acid copolymer membrane has been found to be effective as aproton-conducting solid polymer electrolyte membrane, perfluorinatedsulfonic acid polymer membranes such as ACIPLEX™ perfluorinated sulfonicacid polymer membranes (manufactured by Asahi Glass Co., Japan) andpolymer membranes made by Dow Chemical Co., Japan) and polymer membranesmade by Dow Chemical Co., USA, such as XUS13204.10™ membrane which aresimilar to properties to NAFION™ tetrafluoroethylene-prefluorovinylether sulfonic acid copolymer are also applicable. Membranes ofpolyethylene and polypropylene sulfonic acid, polystyrene sulfonic acidand other polyhydrocarbon-based sulfonic acids (such as membranes madeby RAI Corporation, USA) can also be used depending on the temperatureand duration of fuel cell operation. Composite membranes consisting oftwo or more types of proton-conducting cation-exchange polymers withdiffering acid equivalent weights, or varied chemical composition (suchas modified acid group or polymer backbone), or varying water contents,or differing types and extent of cross-linking (such as cross linked bymultivalent cations e.g., A1 3+, Mg 2+ etc.,) can be used to achieve lowfuel permeability. Such composite membranes can be fabricated to achievehigh ionic conductivity, low permeability for the liquid fuel and goodelectrochemical stability.

As can be appreciated from the foregoing description, a liquid feeddirect oxidation organic fuel cell is achieved using a proton-conductingsolid polymer membrane as electrolyte without the need for a freesoluble acid or base electrolyte. The only electrolyte is theproton-conducting solid polymer membrane. No acid is present in freeform in the liquid fuel and water mixture. Since no free acid ispresent, acid-induced corrosion of cell components, which can occur incurrent-art acid based organic/air fuel cells, is avoided. This offersconsiderable flexibility in the choice of materials for the fuel celland the associated sub-systems. Furthermore, unlike fuel cells whichcontain potassium hydroxide as liquid electrolyte, cell performance doesnot degrade because soluble carbonates are not formed. A solidelectrolyte membrane also minimizes parasitic shunt currents.

Further Improvements. The reactions of the direct methanol/liquid-fedfuel cell are as follows: $\frac{\begin{matrix}{{{{Anode}\quad {CH}_{3}{OH}} + {H_{2}O}} = {{6H^{+}} + {CO}_{2} + {6e}}} \\{{{{Cathode}\quad 1.50_{2}} + {6H^{+}} + {6e}} = {4H_{2}O}}\end{matrix}}{{{{Net}\quad {CH}_{3}{OH}} + 1.50_{2}} = {{CO}_{2} + {2H_{2}O}}}$

The present specification describes various improvements inmanufacturing and forming the preferred structure and materials usedaccording the present invention.

Various experiments carried out by the inventors have ascertained thatone particular preferred catalyst material is platinum-ruthenium(“Pt—Ru”). Various formulations allowing combination of those two metalsare possible. The inventors found that a bimetallic powder, havingseparate platinum particles and separate ruthenium particles produced abetter result than a platinum-ruthenium alloy. The preferred Pt—Rumaterial used according to the present invention has a high surface areato facilitate contact between the material and the fuels. Both platinumand ruthenium are used in the catalytic reaction, and the inventorsfound that it was important that the platinum and ruthenium compounds beuniformly mixed and randomly spaced throughout the material, i.e., thematerial must be homogeneous.

A first aspect of the present invention combines different metals toform a platinum-ruthenium bimetallic powder which has distinct sites ofdifferent materials. While there is some combination between theparticles, the techniques of the present invention ensure that theextent of combination is minimal.

The process of forming the preferred materials is described herein.First, a slurry of platinum salts and ruthenium salts in hydrochloricacid is formed.

A chloroplatinic acid hexahydrate salt H2 PtCl₆. 6H₂O is formed bydissolving chloroplatinic acid crystals in hydrochloric acid.

A ruthenium salt K2RuCl₅. H₂O is formed from potassiumpentachloroaquoruthenium (III).

12.672 grams of chloroplatinic acid crystals are mixed with 13.921 gramsof potassium pentachloroaquoruthenium crystals and 600 ml of 1 molarhydrochloric acid. The mixture of acid and salt is stirred for 15 to 30minutes to obtain a homogeneous mixture.

The acid slurry is then neutralized and precipitated by addition of 140grams of sodium carbonate (Na₂CO₃) per ml per minute at between 20-30°C. During this time, carbon dioxide will vigorously evolve from thesolution. The sodium carbonate is continuously added until the gasevolution ceases. At this time, the solution turns brown-black. Theinventors found that this took about 15 minutes.

Maintaining proper pH during this operation is important—the pH shouldbe maintained at around 9.25 by the slow addition of sodium carbonate.

The “grey powdery mass” is then processed to evaporate water from theslurry. The evaporation takes between 1 and 2 hours and eventually formsa black gluey solid with dry lumps of the material. The black glueysolid is then dried in a vacuum or in flowing nitrogen at 80 to 100° C.A lumpy grey solid is obtained. This solid includes materials which arestill in solution with the sodium chloride.

The chemical content of the grey powdery mass Rutheniumhydroxide—Ru(OH)₃, Platinum hydroxide—Pt(OH)₄ and “gunk” or chlorides,plus excess Na₂CO₃.

The inventors postulate that these extra materials maintain theseparation between the platinum and the ruthenium. If the materials weremaintained alone, they would sinter, causing them to join and increaseparticle size. The carbonate buffer between the particles preventscoalescing.

This lumpy solid material is then reduced in a hydrogen and argonatmosphere to reduce the salt to a metal. The material is transferredinto a glass boat. The boat is placed in the center of a glass tube of atubular furnace. In a gaseous mixture of 7% hydrogen, 93% argon oralternatively in a mixture of hydrogen/nitrogen, the material is reducedat around 225° C. The gas should be flowing over the boat at a rate of50 to 200 ml per minute.

The gas flow is maintained in the heated atmosphere for 14 hours. Then,with hydrogen still flowing over the powder, the catalyst powder isallowed to cool to around 40° C. This forms a mixture of particles ofplatinum, ruthenium, plus other chlorides and carbonates.

The resulting material must then be washed. The material takes severalwashes, e.g. six washes at 60° C. Each wash transfers the sample in theglass boat to a beaker having 1 liter of de-ionized water at 60° C.

Platinum-ruthenium is water insoluble. Hence, the washings do not effectthe platinum ruthenium materials, and only removes the other materials.Each washing includes stirring the water solution for 15 minutes, todissolve the soluble chlorides and carbonates. Since the metal particlesare of submicron size, they do not settle to the bottom, but insteadform a colloidal mixture.

The solution is allowed to cool to 40° C. The solution is latercentrifuged at 3000 rpm for one hour. The centrifuging process leaves aclear supernatant liquid. The supernatant liquid is transferred off, andthe black sediment is transferred to a flask having 1 liter of 60°de-ionized water. This further washing removes any dissolved chlorides.

This washing process is repeated a total of six times. It has been foundthat stirring the water and centrifuging is important for total removalof the chlorides. These chlorides are harmful to catalyst performance.However, the inventors found that these chlorides are a necessary binderto minimize the material coalescing but should be removed later.

After the final centrifuging operation, the powder is transferred to abeaker and dried in a vacuum oven at 60° C. for three hours.Alternatively, the material can be freeze-dried. This results in afree-flowing submicron size active platinum-ruthenium catalyst. It isimportant to note that the dried materials have submicron sizes andhence they can easily become airborne. A submicron mask must be worn toensure safety.

The active catalyst powder has been found to include a homogeneousmixture of submicron size platinum particles and ruthenium particles.There are also some trace residuals of RuO₂, ruthenium oxide, andruthenium alloy.

This powder is used as a catalyst on the anode as described herein.

The platinum salt and ruthenium salt which are the initial products ofthis conversion can also be modified by adding titanium dioxide (TiO₂),iridium (Ir) and/or osmium (Os). These materials can be used to improvethe fuel cell performance at relatively nominal cost.

A comparison with the prior art particles shows the significantadvantages of this process. The prior art particles form 5 micron sizeparticles. These particles included ruthenium dioxide. An analysis ofthe particles of the present invention shows a homogeneous mixture downto the point of micron particle size. Under a scanning electronmicroscope there are no bright and dull spots—all materials appear to betotally grey. This shows that the mixing process has formed a totallyhomogeneous material.

The material prepared according to this process is called anode catalystmaterial. Further processing of this anode catalyst by combining withnafion solution, etc. results in an “ink”. As described herein, thisincludes a combination of platinum metal and ruthenium metal. Theinventors have found the preferred ratio of platinum to ruthenium can bebetween 60/40 and 40/60. The best performance is believed to occur at60% platinum, 40% ruthenium. Performance degrades slightly as thecatalyst becomes 100% platinum. It degrades more sharply as the catalystbecomes 100% ruthenium.

Other additions are added to the salt to improve characteristics and toreplace the catalyst materials by other less-expensive materials. Theinventors realized that this fuel cell must be formed from inexpensivematerial. Unfortunately, platinum is an extremely expensive material. Asof today's writing, platinum-ruthenium is the best material for thecatalyst. The inventors have investigated using replacements for all orpart of the platinum in the catalyst. The substitution is based on theway that the platinum-ruthenium catalyst works.

The reaction which occurs at the anode is CH₃OH+H₂O→CO2+H⁺+e⁻. Theinventors believe that platinum-ruthenium catalyzes this reaction byaiding in disassociating the materials on the catalyst surface. Thematerial draws the electrons out, and allows them to disassociate. Thereaction can be explained as follows.

Methanol is a carbon compound. The carbon atom is bound to four otheratoms. Three of the bonds are to hydrogen atoms. The other bond is to ahydroxyl, OH, group. The platinum disassociates methanol from itshydrogen bonds, to form:

M═C—OH (M is the Pt or other metal site catalyst)+3H⁺. The rutheniumdisassociates the hydrogen from the water molecule (HOH) to form M—OH.These surface species then reassemble as CO₂+6H⁺+6e⁻. The H⁺ (protons)are produced at the anode, and cross the anode to the cathode where theyare reduced. This is called a bifunctional catalyst.

Any material which has a similar function of disassociating the methanoland water as described can be used in place of the platinum. Theinventors have investigated several such materials. They foundalternatives to platinum including palladium, tungsten, Rhodium, Iron,Cobalt, and Nickel which are capable of dissociating C—H bonds.Molybdenum (MoO₃), niobium (Nb₂O₅), zirconium (ZbO₂), and rhodium (Rh)may also be capable of dissociating H—OH as M—OH. A combination of theseare therefore good catalysts. The catalyst for dissociating the H—O—Hbonds preferably includes Ru, Ti, Os, Ir, Cr, and/or Mn.

Ruthenium can be replaced either wholly or partly by a ruthenium-likematerial. The inventors found that iridium has many characteristicswhich are similar to ruthenium. A first embodiment of this aspect,therefore, uses a combination of platinum, ruthenium and iridium in therelative relationship 50-25-25. This adds the salt H₂IrCl₆. H₂O to theinitial materials described above, in appropriate amounts to make a50-25-25 (Pt—Ru—Ir) combination.

It has been found that this catalyst also operates quite well, usingless ruthenium.

Another material which has been found to have some advantages ismaterial including titanium compounds. Any titanium alkoxide or titaniumbutoxide, e.g. titanium isopropoxide or TiCl₄—can also be added to theoriginal mixture. This forms an eventual combination ofplatinum-ruthenium—TiO₂, also formed in a 50-25-25 (Pt—Ru—TiO₂)combination.

Platinum-ruthenium-osmium is also used. Osmium is added to the mixtureas a salt H₂OsCl₆. 6H₂O, and has also been found to produce advantageousproperties.

However formed, these materials used to form the platinum ink must beapplied to the anode. Various techniques can be used to apply thismaterial. Formation of the anode, therefore, is described next.

Carbon paper formation.

Fuel crossover is a source of inefficiency in this fuel cell. Fuelcrossover in this fuel cell occurs when methanol passes through theanode instead of reacting at the anode. The methanol passes through theanode, the membrane electrode assembly, through the membrane and thenthrough the cathode. The methanol may react at the cathode: this lowersthe efficiency of the fuel.

The electrodes of the present invention are preferably formed using abase of carbon paper. The starting material point is TGPH-090™ carbonpaper available from Toray, 500 Third Avenue, New York, N.Y. This paper,however, is first pre-processed to improve its characteristics. Thepre-processing uses a DuPont TEFLON™ 30 polytetrafluoroethylenesuspension of about 60% solids.

The paper can alternately be chopped carbon fibers mixed with a binder.The fibers are rolled and then the binder is burned off to form a finalmaterial which is approximately 75% porous. Alternately, a carbon clothpaper could be used. This will be processed according to the techniquesdescribed herein. Alternately, a carbon paper cloth could be used. Thiswill be processed according to the techniques described herein to form agas diffusion/current collector backing.

The preferably processed carbon paper includes paper within embeddedTEFLON™ polytetrafluoroethylene particles. The spaces between theTEFLON™ polytetrafluoroethylene particles should preferably be smallenough to prevent methanol from passing therethrough. Even bettercharacteristics are used when other methanol derivatives, such as TMMare used. The anode assembly is formed on a carbon paper base. Thiscarbon paper is coated with TEFLON™ polytetrafluoroethylene, meaningthat TEFLON™ polytetrafluoroethylene is added to improve its properties.The inventors have found that there is an important tradeoff between theamount of TEFLON™ polytetrafluoroethylene which is added to the paperand its final characteristics.

It is important to maintain a proper balance of the amount of TEFLON™polytetrafluoroethylene used, as described herein.

The paper is coated with TEFLON™ polytetrafluoroethylene to make itwater repellent, and to keep the platinum ink mix from seeping throughthe paper. The paper needs to be wettable, but not porous. This delicatebalance is followed by dipping and heating the paper. The inventorsfound a tradeoff between the degree of wettability of the paper and theamount of impregnation into the paper, which is described herein.

First, the TEFLON™ 30 polytetrafluoroethylene emulsion must be diluted.One gram of TEFLON™ 30 polytetrafluoroethylene is added to each 17.1grams of water. One gram of TEFLON™ 30 polytetrafluoroethylene of weight60% corresponds to 60 grams of TEFLON™ polytetrafluoroethylene per 100ml. This material is poured into a suitable container such as a glassdish. The carbon paper is held in the material until soaked.

The soaking operation corresponds to weighing a piece of carbon paper,then dipping it into the solution for about 10 seconds or untilobviously wet. The carbon paper is removed from the solution withtweezers, making as little contact with the paper as possible. However,the characteristics of TEFLON™ polytetrafluoroethylene are such that thetweezers themselves will attract the TEFLON™ polytetrafluoroethylene,and cause an uneven distribution of fluid. TEFLON™polytetrafluoroethylene-coated tweezers are used to minimize thispossibility. The carbon paper is held with a corner pointing down, toallow excess solution to drain off.

TEFLON™ polytetrafluoroethylene emulsion's surface tensioncharacteristics are such that if the material were laid on a glasssurface, a lot of the TEFLON™ polytetrafluoroethylene would be draggedout by surface tension. Instead, a paper-drying assembly is formed asshown in FIG. 3. A plurality of TEFLON™ polytetrafluoroethylene-coveredwires 202 are stretched over a catchbasin such as a dish 200. Thestretched wires form two sets of orthogonally-extending supports 202 and204. The carbon paper which has just been treated with TEFLON™polytetrafluoroethylene solution is held across these supports.

Ideally, the wires are TEFLON™ polytetrafluoroethylene-coated wireshaving a diameter of 0.43 inches. While these dimensions are notcritical, a smaller amount of contact with the paper makes thesuspension distribution on the wire more even. Kinks 206 are formed inthe wires to prevent the carbon paper from touching the wires all alongits length and hence further minimize the area of contact.

The paper-drying assembly shown in FIG. 3 is then placed into an oven at70° C. for one hour. The treated carbon papers are removed from the dishafter drying, and placed into glass containers. These are then sinteredin a furnace oven at 360° C. for one hour. A properly processed paperwill have its weight increased by 5% over the course of this process.More generally, any weight increase between 3 and 20% is acceptable. Thepaper is weighed to determine if enough absorption has occurred and/orif further paper processing will be necessary.

This substrate plus a catalyst layer forms the eventual electrode.

Two preferred techniques of application of the catalyst including layerare described herein: a direct application and a sputtering application.Both can use the special carbon paper material whose formation wasdescribed above, or other carbon paper including carbon paper which isused without any special processing. The direct application technique ofthe present invention mixes materials with the platinum-rutheniummaterial described above or any other formulation, more generally,catalyst materials. The catalyst materials are processed with additionalmaterials which improve the characteristics.

Platinum-ruthenium powder is mixed with an ionomer and with a waterrepelling material. The preferred materials include a solution ofperfluorsulfonic acid NAFION™ tetrafluoroethylene-perfluorovinyl ethersulfonic acid copolymer and TEFLON™ polytetrafluoroethylenemicro-particles. 5 grams of platinum-ruthenium powder are added per 100ml of NAFION™ tetrafluoroethylene-perfluorovinyl ether sulfonic acidcopolymer in solvent.

A DuPont T-30 mix of 60% TEFLON™ polytetrafluoroethylene solid by weightappropriately diluted is added. These TEFLON™ polytetrafluoroethylenemicro-particles are then mixed. Preferably, a dilute TEFLON™ 30polytetrafluoroethylene suspension of 12 weight percent solids including1 gram of TEFLON™ 30 polytetrafluoroethylene concentrate to 4 grams ofde-ionized water is made. 300 mg of de-ionized water is added to 350 mgof the 12 weight % TEFLON™ polytetrafluoroethylene solution describedabove. 144 mg of platinum-ruthenium is mixed to this solution. Theresultant mixture is then mixed using an ultrasonic mixingtechnique—known in the art as “sonicate”. The ultrasonic mixing ispreferably done in an ultrasonic bath filled with water to a depth ofabout ¼ inch. The mixture is “ultrasonicated” for about 4 minutes.

It is important that the TEFLON™ polytetrafluoroethylene must first bemixed with the platinum-ruthenium as described above to form about 15%by weight TEFLON™ polytetrafluoroethylene. Only after this mixture ismade can the NAFION™ tetrafluoroethylene-perfluorovinyl ether sulfonicacid copolymer be added. The inventors have found that if NAFION™tetrafluoroethylene-perfluorovinyl ether sulfonic acid copolymer isadded first, it may surround the particles of platinum and ruthenium.Therefore, the order of this operation is critically important. At thispoint, 0.72 grams of 5 weight % NAFION™tetrafluoroethylene-perfluorovinyl ether sulfonic acid copolymer isadded to the jar, which is sonicated again for 4 minutes. Moregenerally, approximately 1 mg of NAFION™tetrafluoroethylene-perfluorovinyl ether sulfonic acid copolymer needsto be added per square cm of electrode to be covered. The amount ofTEFLON™ polytetrafluoroethylene described above may also be modified,e.g. by adding only 652 ml of the solution.

This process forms a slurry of black material. This slurry of blackmaterial is then applied to the carbon paper. The application can takeany one of a number of forms. The simplest form is to paint the materialon the carbon paper backing, using alternating strokes in differentdirections. A small camel hair brush is used to paint this on. Thepreferred material amounts described above, to form enough catalyst forone side of a 2-inch by 2-inch piece of 5 weight % coated with TEFLON™polytetrafluoroethylene carbon paper. Accordingly, the painting iscontinued until all the catalyst is used.

A drying time of two to five minutes between coats should be allowed, sothat the material is semi-dryed between coats and each coat should beapplied in a different direction. The anode needs to then dry for about30 minutes. After that 30 minutes, the anode must be “pressed”immediately. The pressing operation is described herein.

The resulting structure is a porous carbon substrate used for diffusinggases and liquids, covered by 4 per square cm of catalyst material.

An alternative technique of applying the materials sputters thematerials onto the backing.

We have now described how to form the anode. Next, the techniquesinvolved in forming the preferred proton conducting membrane (theNAFION™ tetrafluoroethylene-perfluorovinyl ether sulfonic acidcopolymer) and then the techniques in forming the cathode will bedescribed.

Proton Conducting Membrane—The preferred material described herein iscoated with TEFLON™ polytetrafluoroethylene. However, other materialscan also be used to form proton conducting membranes. For example, otherperfluorsulfonic acid materials can be used. It is postulated thatdifferent materials with carboxylic acid groups can also be used forthis purpose.

The preferred embodiment starts with coated with TEFLON™polytetrafluoroethylene, available from DuPont. This material is firstcut to the proper size. Proper sizing is important, since the endmaterials will be conditioned. First, the NAFION™tetrafluoroethylene-perfluorovinyl ether sulfonic acid copolymer isboiled in a hydrogen peroxide solution. A 5% solution of hydrogenperoxide is obtained, and the membrane is boiled in this solution for 1hour in 80-90° C. This removes any oxidizable organic impurities.

Following this peroxide boiling step, the membrane is boiled inde-ionized water, at close to 100° C., for 30 minutes. Hydrogen peroxidewhich was previously adsorbed into the membrane is removed along withother water-soluble organic materials.

The thus-processed membrane is next boiled in a sulfuric acid solution.A one molar solution of sulfuric acid is prepared by dilutingcommercially available 18-molar concentrated ACS-grade sulfuric acid.The ACS-grade acid should have metal impurities in an amount less than50 parts per million. The membrane is boiled in the 1-molar sulfuricacid at about 100° C. to more completely convert the material into aproton conducting form.

The processed material is next boiled in de-ionized water at 90-100° C.for thirty minutes. The water is discarded, and this boiling step isrepeated three more times to purify the membrane.

After these washings, the membrane is free of sulfuric acid and incompletely “protonic” form. The membrane is stored in de-ionized waterin a sealed container until it is ready for further processing.

Cathode construction. The cathode is constructed by first preparing acathode catalyst ink. The cathode catalyst ink is preferably pureplatinum, although other inks can be used and other materials can bemixed into the ink as described herein. 250 mg of platinum catalyst ismixed with 0.5 gram of water including 37-½ mg of TEFLON™polytetrafluoroethylene. The mix is sonicated for five minutes andcombined with a 5% solution of NAFION™tetrafluoroethylene-perfluorovinyl ether sulfonic acid copolymer. Themix is again sonicated for five minutes to obtain a uniform dispersal.This forms enough material to cover one piece of 2×2″ carbon paper.Unprocessed TORAY™ carbon fiber paper can be used with no TEFLON™polytetrafluoroethylene therein. However, preferably the material iscoated with TEFLON™ polytetrafluoroethylene as discussed above. Theprocedures are followed to make a 5% TEFLON™ polytetrafluoroethyleneimpregnated paper. The paper is then heated at 300° C. for one hour tosinter the TEFLON™ polytetrafluoroethylene particles. Catalyst ink isthen applied to the paper as described above to cover the material with4 mg/cm²/g of PT. TEFLON™ polytetrafluoroethylene content of thepapercan vary from 3-20%, 5% being the preferred.

Sputtering

An alternative technique of cathode forming forms a sputtered platinumelectrode. This sputtered platinum electrode has been found to havesignificant advantages when used as a plain air electrode. The processof forming the sputtered platinum electrode is described herein.

The cathode electrode carries out a reaction of O₂+H⁺+e⁻→water. The O₂is received from the ambient gas around the platinum electrode, whilethe electron and protons are received through the membrane. Thisalternative technique for forming the cathode electrode starts with fuelcell grade platinum. This can be bought from many sources includingJohnson-Matthey. 20 to 30 gms per square meter of surface area of thisplatinum are applied to the electrode at a particle size of 0.1 to 1micron.

A platinum source is a solid rod of material. According to thisembodiment, the material is sputtered onto the substrate prepared asdescribed above. The platinum powder is first mixed with aluminumpowder. This mixing can be carried out using mechanical means forexample, or it can be done using salt combination techniques asdescribed above for the formulation of the anode ink. Theplatinum-aluminum mixture is sputtered onto the carbon paper backingusing any known sputtering technique from the semiconductor arts.

The platinum is sputtered as follows using the system diagrammed in FIG.4. A standard 4-inch target 250 holds the carbon paper electrode 252.The target is rotated by a motor 254 at one rotation between per 10seconds. The preferred technique used herein sputters platinum from afirst Pt source 260 and aluminum from an Al source 262. The platinum issputtered to 0.23 amps and the aluminum at 0.15 amps at around 200volts. The two sources impinge from different directions in oppositesides towards the targets at 45° angles.

The inventors found that 20 Torr was the ideal pressure for thissputtering, although any pressure between 1 to and 50 to could be used.The Argon pressure is about 30 mtorr. However, different argon pressurescan be used to form different particle sizes. The sputtering is done forabout 8 minutes.

Preferably, once sputtered, the etching is carried out by dipping thesputtered backing into an etching solution, followed by a washingsolution followed by dipping.

The sputtered electrode is a mixture of Al and Pt particles on thebacking. The electrode is washed with potassium hydroxide (KOH) toremove the aluminum particles. This forms a carbon paper backing withvery porous platinum thereon. Each of the areas where the aluminum wasformed is removed—leaving a pore space at that location. The inventorsfound that a thick coating of the Pt—Al material would prevent washingout the Al from some lower areas of the catalyst. The present inventionuses a thin coating—preferably a 0.1 micron coating or less with amaterial density between 0.2 mg per cm² and 0.5 mg per cm².

At this point in the processing, we now have an anode, a membrane, and acathode. These materials are assembled into a membrane electrodeassembly (“MEA”)

MEA Formation

The electrode and the membranes are first laid or stacked on a CP-grade5 Mil, 12-inch by 12-inch titanium foil. Titanium foil is used by thepresent inventors to present any acid content from the membrane fromleaching into the foil.

First, the anode electrode is laid on the foil. The proton conductingmembrane has been stored wet to maintain its desired membraneproperties. The proton conducting membrane is first mopped dry to removethe macro-sized particles. The membrane is then laid directly on theanode. The cathode laid on top of the membrane. Another titanium foil isplaced over the cathode.

The edges of the two titanium foils are clipped together to hold thelayers of materials in position. The titanium foil and the membranebetween which the assembly is to be pressed includes two stainless steelplates which are each approximately 0.25 inches thick.

The membrane and the electrode in the clipped titanium foil assembly iscarefully placed between the two stainless steel platens. The twoplatens are held between jaws of a press such as an arbor press or thelike. The press should be maintained cold, e.g. at room temperature.

The press then actuated to develop a pressure between 1000 and 1500 psi,with 1250 psi being an optimal pressure. The pressure is held for 10minutes. After this 10 minutes of pressure, heating is commenced. Theheat is slowly ramped up to about 146° C.; although anywhere in therange of 140-150° C. has been found to be effective. The slow ramping upshould take place over 25-30 minutes, with the last 5 minutes of heatingbeing a time of temperature stabilization. The temperature is allowed tostay at 146° C. for approximately 1 minute. At that time, the heat isswitched off, but the pressure is maintained.

The press is then rapidly cooled using circulating water, while thepressure is maintained at 1250 psi. When the temperature reaches 45° C.,approximately 15 minutes later, the pressure is released. The bondedmembrane and electrodes are then removed and stored in de-ionized water.

Flow Field. A fuel cell works properly only if fuel has been properlydelivered to the membrane to be reacted an/or catalyzed. The membraneelectrode assembly of the present invention uses a flow field assemblyas shown in FIG. 5. Each membrane electrode assembly (“MEA”) 302 issandwiched between a pair of flow-modifying plates 305 and 312 whichinclude biplates 304,305 and end plates 312. A flow of fuel isestablished in each space 303 between each biplate/endplate and MEA. Thecollection of biplates/endplates and MEA forms a “stack”. The biplateincludes provisions for fluid flow at both of its oppositely-facingsurfaces. The end flowplate of the stack is an end plate 312 instead ofa biplate. The endplate has chambers on one side only. The biplate 305includes a plurality of separators 306 and a plurality of chamberforming areas 308. The separators 306 have the function of pressingagainst the membrane electrode assembly 302. The end surface ofseparators 306 are substantially flat surfaces 315 that contact thesurface of the MEA 302.

The biplates are formed of an electrically conductive material in orderto couple all the membrane electrode assemblies 302, 310 in series withone another.

Membrane electrode assemblies 302, as described above include an anode,a membrane, and a cathode. The anode side 318 of each membrane electrodeassembly is in contact with an aqueous methanol source in space 314. Thecathode side of each membrane electrode assembly is in contact with anoxidant air source which provides the gaseous material in space 316 forthe reactions discussed above. The air can be plain air or can beoxygen.

Flows of these raw materials are necessary to maintain proper supply offuel to the electrode. It is also desirable to maintain the evenness ofthe flow.

One stack design of the present invention uses the system shown in FIG.6. The fuel is supplied from fuel supply chamber 602, which is typicallya high volume element which includes fuel under pressure. Narrownozzle-like elements 606 cause a large pressure drop therealong. Thepressure drop in the thin line is much greater than any pressure dropalong the supply. This evens the flow within the cells and among thecells.

A careful trade-off must be balanced between the amount of surfaceacting as a pressing element and the amount of surface that acts as aholding element.

It is desirable to apply even pressure against the membrane electrodeassembly 202 from both sides for many reasons. However, in places wherethe pressing surface 306 presses against the membrane, the membraneelectrode assembly 302 cannot be directly in contact with the methanol.Instead it is pressed by the surface 315. Therefore, that part of thesurface of MEA 302 does not react. The different designs according tothe present invention carry out various functions to improve the flow orimprove some characteristic of its reliability.

Each of the nozzles 606 has a narrow width. The outlet 605 of eachnozzle 606 faces one island pressing area 608 which corresponds to apressing surface 306. The supply of fuel from nozzle 606 is supplieddirectly against the interface surface 610 of island 608. The islands608 in FIG. 6 are rectangular in shape. Interface surface 610 is anarrow side of the rectangular island 608. The wider side of the island608 is parallel to the flow. All input flows face directly against oneof the surfaces of an island.

The inventor found that this preferred narrow layout creates turbulencein the area of the islands 608. The turbulence stirs the fuel in thechamber and forms a more even flow through the system. This turbulencealso facilitates flow between each of the islands 608. The output flowis eventually received by output nozzles 612 and routed into outputconduit 614. The output nozzles are analogously placed adjacent surfaces620 of the islands 608, thus causing further turbulence.

The islands according to this embodiment are 50 mil on the interfaceside 610, and 150 mil on the wider side. The pressure drop across thestack is approximately 0.06 psi for the stack.

Other biplate configurations can also be used.

It is important that the biplates themselves be lightweight and thin, toallow increase of the stacking pitch as much as possible.

Graphite is difficult to machine, and is relatively thick. Graphite'sadvantages include its imperviousness to liquid and gas.

A number of alternative solutions are used according to the presentinvention. A first modification of the present invention uses the systemshown in FIG. 7.

Interface layer 702 forms a dense, conductive liquid gas imperviouslayer. This reduces the amount of fuel, gas or liquid which can crossthe biplate assembly over the materials. However, a dense porousmaterial is used as the crossing areas 700. The porosity allows acertain amount of the material to seep into the MEA through thisinterface layer.

The dense porous material can be conductive carbon, for example, whichis much easier to machine than graphite. The seepage is stopped byinterface material, which prevents the liquid and gas from crossingacross the whole biplate.

The porosity of the pressing parts allows the liquid and gas to reachsome of the parts of the membrane electrode assembly which are beingpressed by the pressing element. The methanol hence may penetrate tothese areas which would otherwise be less efficiently convecting.

The central binding layer 704 is low-density (“LD”) carbon. LD carbon isrelatively easy to work with and inexpensive. Since the LD carbon iscovered at all locations by graphite, however, its undesirablecharacteristics are mostly masked.

A second embodiment used to form a biplate is shown in FIG. 8. Thissecond biplate embodiment uses a layered titanium-carbon ultrathinbiplate. Any biplate should be thin; because we want the stack to be asthin as possible for any given voltage. Each membrane electrode assemblyand biplate will produce a voltage when energized, we call that theinherent voltage. The inherent voltage, and the thickness of the device,sets the maximum possible volts-per-inch of thickness of the device ofthe present invention. One important component of thevolts-per-thickness is the thickness of the biplate.

FIG. 8 shows the second biplate embodiment of the present invention.This material uses a layered concept to form a biplate combining thebest characteristics of the materials. A titanium carbide interfacelayer 800 is bonded to titanium bonding layer 802. The titanium bondinglayer 802 is preferably 3 mils thick. These two layers together preventmigration of any protons across the biplates and also ensure adequateelectrical bonding characteristics. The titanium materials are coveredwith separating materials 804 which include surfaces to hold thebiplates in place. A certain measure of porosity is hence enabled as inthe FIG. 7 embodiment.

Of course, the titanium could be replaced by any metal with similarconducting and chemically stable characteristics.

The inventors of the present invention recognize that the graphitematerial usually used must represent a trade-off between the competingnecessities.

Efficiency of operation requires that fuel from one side of one biplate,e.g. the anode side, not seep across to to the other side of the samebiplate, which interfaces to a cathode. If the biplate were porous, thefuel materials could seep across. However, since no fluids can passthrough the biplates, this has meant that no fluids can reach theportions of the electron membrane assembly being pressed by the pressingsurfaces biplates, e.g. 306. Therefore these portions of the membraneelectrode assembly which are being pressed by those pressing surface arenot efficiently producing electrical activity. This lowers the overallefficiency of the cell.

These embodiment of the present invention provide a new kind oftrade-off. The membrane electrode assembly is pressed by a porousportion of the biplate. This porous portion allows at least some of thefuel to reach that portion of the electrode. This can improve theelectrical operation of the MEA. This feature of the present inventionalso provides other bonded pieces which prevent the fluid from passingover into the other portions of the electrode membrane assembly.

System. The basic system of the present invention is shown in FIG. 9.The system is based on the inventor's recognition of ways of recyclingthe output of the fuel cell. The fuel cell consumes methanol or methanolderivatives, water and produces output products including methanol orderivatives, water, and gases. Methanol represents the fuel that is tobe consumed. Any fuel cell system would need to carry quantities ofmethanol fuel to be consumed. However, this reaction would also requireequal amounts of water. The inventors recognized that the water used inthe reaction can be recycled from the cathode.

The amount of power that a vehicle can produce is limited by itspayload—i.e. the weight of the vehicle and its occupants. All vehiclesare limited in power by the amount of weight that they must carry. Moreweight limits a vehicle's power and hence makes the vehicle lessefficient. For example, a passenger car usually does not hold more than20-30 gallons of gasoline. This has been determined by many to representan optimum trade-off between the distance that the vehicle can runbefore re-filling the tank, and the excess weight that would result froma larger fuel tank.

Vehicle engineers decide how much payload weight they are willing toallow. The inventors describe techniques which ensure that this weightis taken up by fuel, not water.

One of the features of the system of the present invention is tomaintain the water balance so that most of the water is recycled and nosubstantial source of water needs to be carried.

The overall system is shown in FIG. 9. Methanol tank 900 stores puremethanol (or other methanol-type derivative fuel). A first liquid pump902 pumps the methanol through a valve 904 to a circulation tank 906.Water tank 908 provides a supply of water where necessary. The water ispumped by pump 910 through valve 912 to recirculation tank 906. Acentral controller 914 controls the overall operation of the entiresystem. Controller 914 controls the relative positions of the valves 904and 912.

Methanol concentration sensor 916 is preferably located either in themethanol or very close to it. Methanol sensor 916 detects theconcentration of methanol in the circulation tank, and controller 914uses this information to control further operation of the system.

The aqueous methanol in the circulation tank is maintained by thiscontrol system at 1-2 M. Therefore, the methanol in line 918 should alsobe of the proper concentration. Pump 920 pumps the methanol through afuel filter 922 to the membrane electrode stack 924. The stack usedherein can be a similar stack to those described previously. Theelectrical output 926 of the stack 924 is sent to the motor to drive thepayload and also drives controller 914 and other electrical systems suchas the compressor 930.

The stack is also driven with inlet air 932 through the compressor 930.Air filter 934 cleans the air prior to its introduction into the stack.

The fuel out of the stack includes two components: water and methanol.Both components are processed using respective condensers 940 and 942 tolower the water temperature sufficiently to allow both the methanol andthe water to condense. Fans 944 may be used to facilitate this cooling.The recycled methanol and water are both resupplied to the circulationtank. The recycled methanol 946 from the output of the methanol stack,and the recycled air and water from the inlet air 952 recycle intocirculation tank 906.

Fluid engineers have recognized that pumping gas is extremely expensivein terms of energy resources, while pumping liquid is extremelyinexpensive. One aspect of the present invention may requirepressurizing the air to the cathode. For example, the air may need to bepressurized to 20 psi. However, the output air on line 945 (afterreacting with the cathode) will be almost as highly pressurized. Thisoutput air 944 will be pressurized to 19 psi. Accordingly, the outputair 945 is coupled to a pressure-driven turbine 947. This expander isrun by pressure, and used to drive the air compressor 930. Without thisrecycling of the pressurized power, the air compressor might use as muchas 20-30% of the power produced by the cell.

Expander output 948 includes an air and water combination. This waterand air is separated to vent the exhaust air at 950, and the recycledwater being returned to the circulation tank 902. A vent for excesswater 954 may also be necessary. This vent is controlled by controller914, and necessary at some time if too much water is being recirculated.

As an alternative to the sensor and controller, the amount of fuel whichis supplied can be metered. The fuel cell first starts its operation atroom temperature. However, the current fuel cell is intended to operateat about 90° C. The electrochemical fuel cell reaction will eventuallyheat up the fuel cell to the proper temperature.

The present invention operates using methanol sensors. A particularlypreferred methanol sensor uses MEA technology described above. Asdescribed above, a fuel cell is formed of an anode and a cathode. Theanode receives methanol. The cathode receives air or oxygen.

This sensor uses the modified fuel cell shown in FIG. 10. A Pt—Ru anode1002 is connected to a NAFION™ tetrafluoroethylene-perfluorovinyl ethersulfonic acid copolymer electrolyte 1004, which is connected to a Ptcathode. The cathode is preferably larger than the anode, e.g., thecathode is three times the area of the anode.

The cathode 1006 (and anode) are immersed in the methanol solution.Therefore, since the cathode 1006 is under fluid, it cannot react withair, and hence the H₂ cannot react to H₂O as in the basic fuel cellreaction. Applying a voltage to the fuel cell changes, e.g. reverses,the reaction which occurs. Under current, the anode reacts directly withmethanol to form CO2, and the cathode will change protons to hydrogen. Asmall cathode, and a large anode to reduce protons, further enhances thesensitivity of this methanol electrode.

The reactions, therefore, include:

(+) H₂O+CH₃→CO₂+6H⁺+6e⁻

(−) 2H⁺+2e⁻→H₂

A constant voltage is applied by constant voltage circuit 1010. Ammeter1012 measures the current. FIG. 11 shows the relationship between thecurrent and the methanol concentration in the solution. Controller 1014,which can be a process controller or microprocessor, looks up theclosest methanol concentration corresponding to the measured current,using the plotted FIG. 11 relationship.

Since the FIG. 11 plot may be highly temperature dependent, thermocouple1016 may provide correction information.

Another important feature of the present invention is related topractical use of this system in an automotive environment. Practical usewould require delivery of methanol from the methanol equivalent of a gaspump. Methanol would have hydrocarbon impurities when taken from the gaspump. Such impurities would be very dangerous to the system described bythe present invention which requires highly pure methanol. According tothe present invention, a fuel filter is used. The fuel filter is shownin FIG. 12. A three stage filter including zeolite crystals therein ofthe synthetic 25M (Mobil) types or the natural types. Typically azeolite acts as a molecular sieve. The zeolite crystals are used tofilter the methanol to remove any hydrocarbon impurities therefrom.These zeolites can include a set of layers of three or more with poresizes varying from 3-10 Å gradually from layer 1-3. Layer 1 is typicallythe large pore diameter zeolite, offerite, to remove large molecules.Mordenite, a natural zeolite, is used in layer 2 to exclude n-paraffins,n-butanes and n-alkanes. Zeolite 3A or 4A can be used to remove smallmolecules such as propane and ethane in layer 3. This preferably forms agraded molecular sieve.

Mono-polar approaches.

Previous approaches to fuel cells used a number of fuel cells in series.The series connection of fuel cells adds the output voltages to form ahigher overall allowed the output of the stack to be increased to ahigher and more usable voltage. The inventor of the present inventionrealized, moreover, certain advantages which can be obtained from usinga non stacked approach, which the present inventor has labelledmono-polar. This monopolar approach maintains each membrane electrodeassembly completely separately from all the others. This completelydifferent approach allows each element of the assembly to be made muchlarger, and with a better efficiency. However, we can only get a loweroutput voltage. Each mono-polar element can be assembled into a stack.The important thing according to this feature is that each membraneelectrode assembly is separately connected,and the seperately connectedelements are connected in series, rather than assembling them into astack.

A first embodiment of the monopolar invention is shown in FIG. 13. Thisembodiment could be used to form a fuel cell that does not requirecontact forces in order to make electrical connections. Membrane 1300 ispreferably a Nafion membrane. The NAFION™tetrafluoroethylene-perfluorovinyl ether sulfonic acid copolymermembrane includes a central area with a termination of metal clothstrips 1302, eg a screen. The metal cloth or screen 1302 is covered withappropriate catalysts of the types described above. Current carryingtabs 1304 bring the voltage which is produced to the outside.

A plastic or metal flow field insert 1306 channels the appropriate fuelmaterial to the respective side of the catalyst-covered cloth. Flowfield element 1308 can be located on the other side.

The material with the catalyst thereon is therefore attached to theNAFION™ tetrafluoroethylene-perfluorovinyl ether sulfonic acid copolymerbacking and pressed thereagainst to form a fuel cell in a similarelectrical but different mechanical way.

FIG. 14 shows a cross-section of the device. The tabs 1304 conduct theelectricity to an electrode area 1400. Methanol is brought into amethanol chamber 1402, into a sealed area on a first side of themembrane. The seal is maintained by a ring sealing area 1406. Air isconducted to a second side of the membrane through air chamber 1408,which is similarly sealed on the other side. Each of these elementsoperates as a stand-alone unit, independent of the other units. Thecurrent from these elements can be connected in series to provide ahigher voltage.

A second alternative embodiment of the invention is shown in FIG. 15.This embodiment uses a membrane 1500, along with a titanium sheet 1502.Titanium cloth 1504 is spot-welded to the titanium sheet. The titaniumcloth 1504 acts as the cathode and may be coated with platinum. Titaniumcloth 1506 acts as the anode and may be coated with appropriate platinumruthenium.

A gasket and bonding ring 1508 forms a chamber 1510 between the membraneand the anode. In a similar way, another gasket and bonding ring 1510forms a chamber between the membrane and the cathode.

The titanium sheet has a bead seal 1512 thereon to maintain the chamber.Voltage produced by the titanium sheet is coupled to the current takeoffarea 1514.

This embodiment also includes places for rivets or fasteners, since thebead sealing would allow metal fasteners to be used. This integratedsystem could be extremely thin, especially if titanium foil were used.

The elements could be used in a device shown in FIG. 16. Each of thesedual cell modules shown in FIGS. 13 or 15 includes a cathode and anodethereon. The elements shown in FIG. 16 are assembled to form twoadjacent anodes up to cells 1602 and 1604 which face one another. A flowfield 1606 is established between the anode 1602 and 1604. This flowfield should include an air flow there between. In a similar way, twoadjacent cathode face one another and a flow field 1608 is formedtherebetween to include the appropriate airflow therebetween.

FIG. 17 shows an expanded view of how these cells would be used. Flowfield 1700 is an airflow field which faces the cathode side 1702 offirst cell 1704. The anode side 1706 faces a second, methanol flow field1708. Methanol is input through methanol input port 1710 and out throughoutput port 1712. The methanol flow field also faces the anode side 1714of a second bipolar cell 1716. The cathode side 1720 of the secondbipolar cell 1716 faces another flow field element 1722 for air.

Although only a few embodiments have been described in detail above,those having ordinary skill in the art will certainly understand thatmany modifications are possible in the preferred embodiment withoutdeparting from the teachings thereof.

All such modifications are intended to be encompassed within thefollowing claims.

What is claimed is:
 1. A methanol concentration sensor device,comprising: an anode, cathode, and an electrolyte element, includingelectrical connections thereto, and adapted to be maintained in amethanol solution; a source of electrical power; and a meter, connectedto said source of electrical power, and measuring an amount of currentflowing from said source, representing a concentration of methanol inthe methanol solution.
 2. A device as in claim 1 wherein said cathode isphysically larger in size than said anode.
 3. A device as in claim 2wherein said cathode has an area which is three times the area of saidanode.
 4. A device as in claim 1 further comprising an element whichdetects temperature, said temperature used for determining correctioninformation for said concentration.
 5. A method of detecting methanolconcentration, comprising: immersing an anode, a cathode, and anelectrolyte in a methanol solution; providing a constant voltage acrosssaid anode to cathode; and detecting a current which flows across saidanode to cathode, said current being proportional to concentration ofmethanol.
 6. A method as in claim 5 wherein said cathode is larger thansaid anode.
 7. A method as in claim 5 further comprising sensing atemperature of the methanol solution, and using said temperature tocorrect for temperature variations.